This invention relates generally to crystal oscillator circuits, and more particularly the invention relates to a crystal oscillator circuit having sinusoidal to square wave buffering with reduced power consumption and controlled oscillation start-up and steady state operation.
FIG. 1 illustrates an oscillator circuit in an RF signal transmitter and receiver circuit. The crystal 10 is driven by a current source 12 with the current source receiving the oscillator output in a feedback loop 14 and controlled by a bias current 16 applied to current source 12. The output of crystal 10 (e.g., 12 MHz) is applied through phase lockloop 18 to a local oscillator 20 having a higher frequency output (e.g., 2.4 GHz). A loop filter 22 provides feedback from local oscillator 22 to PLL18. The output of local oscillator 20 is applied to mixers 24,26 which respectively step down the frequency of received radio signal or step up the frequency of a transmission radio signal through amplifiers 28,30 and switch/filter 32 to antenna 34.
Operation of the crystal circuit depends on a transconductance device in the oscillator circuit. FIG. 2 is a schematic of a basic 3 point oscillator in which current source 42 passes current through a transconductance device or transistor 40 with the transconductance depending on current magnitude, which in turn controls operation of crystal 10. A paper which describes the theory of operation of the 3 point oscillator is Vittoz et al., xe2x80x9cHigh Performance Crystal Oscillator Circuits: Theory and Applicationxe2x80x9d, IEEE Journal of Solid State Circuits, Volume 23, No. 3, June 1988 (pp. 774-783). As there described, crystal oscillation depends on the transconductance of transistor 40 as illustrated by the complex plane representation of the 3 point oscillator shown in FIG. 3. As described, Z(c) is impedance looking into the circuit from the crystal. From this impedance circle, the conditions for start-up is to have transconductance (gm) set between the gm critical value and the gm max value and preferably at gin optimum. In an ideal environment, there are two modes of operation. For start-up, gm is set to the optimum value on the Z(c) circle. This will result in a maximum negative resistance seen by the crystal for oscillation amplitude buildup. Once amplitude increases to a pre-determined level, transconductance must be reduced in order to maintain the amplitude. Thus in preferred operation, the transconductance of the oscillator must be varied from start-up to steady safe conditions.
Referring again to FIG. 1, PLL 18 requires a low jitter square wave reference signal from the crystal 10. This necessitates a buffer for wave shaping. Further, the crystal oscillator circuit requires control of drive current so that the oscillator start-up can be accelerated but with drive current reduced once a desired amplitude level is obtained.
In accordance with the invention a buffer circuit is provided for use in an oscillator circuit in driving a phase locked loop. The oscillator receives a sinusoidal wave and provides a square wave with reduced jitter and power consumption.
More particularly, two buffers are driven in push-pull fashion and drive a third buffer. The first two buffers have duty cycles set such that there is no overlap of the buffer outputs. In a preferred embodiment, CMOS circuitry is employed in the buffer circuits for reduced power consumption. A complimentary transistor pair functions as an image resistor to isolate noise in a bias voltage branch from the buffer gain stage.
Circuitry is provided to optimize oscillator start-up time with minimal variation across processing, temperature, and supply voltage variations. The circuitry responds to a start-up bias current for setting transconductance in the oscillator circuit at an optimum value with the circuitry having feedback to control transconductance as the oscillation voltage increases, thus preventing a runaway oscillation condition.
The invention and objects and features thereof will be more readily apparent from the following detailed description and dependent claims when taken with the drawings.